Show alert message when archive failure occurs

I recently integrated bundle_cache gem with our CI setup, and it works great! :+1: 

However, I made a mistake that I forgot to add `--path=~/.bundle` into `.travis.yml` and spent some time finding out this problem. (No error messages shown and empty archive uploaded)

Thus I added a kindly reminder for those who make the same mistake, and also make any other archive issue not  ignored.